1. Field of the Invention
The present invention relates to an ultrasonic sensor and a self diagnostic method of the ultrasonic sensor.
2. Description of the Related Art
Conventionally, an ultrasonic sensor includes an ultrasonic-wave detection element attached to a transmission member that transmits an ultrasonic wave. For example, the transmission member is made of metal or resin. The ultrasonic sensor sends an ultrasonic wave from a sending device and receives the ultrasonic wave reflected by an external object by using a receiving device. Thereby, the ultrasonic wave detects a location of the object, a distance to the object, a two-dimensional shape of the object, or a three-dimensional shape of the object. The ultrasonic sensor can be suitably used for a surveillance system of a vehicle. However, the ultrasonic sensor attached to the vehicle may be damaged by an external factor such as an impact. When the ultrasonic sensor is damaged, the surveillance system may malfunction and may be difficult to detect a people or an obstacle with a high degree of accuracy.
US 2006/0196272 A (corresponding to JP-2006-242650A) discloses a self diagnostic method of an ultrasonic sensor that includes a plurality of receiving devices. The ultrasonic sensor self-diagnoses based on a difference in intensity of detection signals of the receiving devices at a time where the receiving devices detect an ultrasonic wave reflected by an external object. Because the present self-diagnostic method uses the ultrasonic wave reflected by the external object, the ultrasonic sensor cannot detect a malfunction when there is no object in a range of detection. For example, when the vehicle is parked in a garage or parking space at which there is no object in front thereof, the ultrasonic sensor cannot detect a malfunction.